The German Technical Rule for Plant Safety (TRAS) No. 410 defines deflagration as follows:
“Deflagration is a reaction which can be triggered in a localized fashion in a prescribed amount of material and which propagates automatically in the form of a reaction front from there through the entire amount of material. The propagation velocity of the reaction front is lower than the speed of sound in the material. Large amounts of hot gases can be liberated in deflagration and these are sometimes also combustible. The deflagration velocity also increases with the temperature and generally also with the pressure”.
Solids capable of deflagration decompose after local action of a sufficiently strong source of ignition (initiation) even without the presence of atmospheric oxygen. In contrast to a fire or explosion, deflagration cannot be prevented by exclusion of oxygen. The measure of making inert with nitrogen or other inert gases which is known from explosion protection offers no protection against deflagration. Processing under reduced pressure has hitherto not been considered to be a protective measure for the processing and handling of materials capable of deflagration.
Explosions are rapid deflagrations with a sudden increase in pressure and temperature. When the speed of sound is exceeded, a deflagration changes into a detonation.
Materials capable of deflagration are usually organic or inorganic compounds in solid form. In particular, organic compounds having functional groups such as carbon-carbon double and triple bonds, e.g. acetylenes, acetylides, 1,2-dienes; strained ring compounds such as azirines or epoxides, compounds having adjacent N atoms, e.g. azo and diazo compounds, hydrazines, azides, compounds having adjacent O atoms, e.g. peroxides and ozonides, oxygen-nitrogen compounds such as hydroxylamines, nitrates, N-oxides, 1,2-oxalates, nitro and nitroso compounds; halogen-nitrogen compounds such as chloramines and fluoramines, halogen-oxygen compounds such as chlorates, perchlorates, iodosyl compounds; sulphur-oxygen compounds such as sulphonyl halides, sulphonyl cyanides, and compounds having carbon-metal bonds and nitrogen-metal bonds, e.g. Grignard reagents or organolithium compounds can undergo deflagration. However, many other organic compounds without the abovementioned functional groups and many inorganic compounds can be capable of deflagration.
Essentially, all materials having a decomposition enthalpy of greater than or equal to 500 J/g are considered to be potentially capable of deflagration. Materials which have a decomposition enthalpy of 300-500 J/g and are capable of deflagration are also known.
The deflagration capability of a substance has to be determined individually in the particular case.
Various test methods for testing the deflagration behaviour of materials and mixtures are known.
In the UN testing handbook “Transportation of Dangerous Goods, Manual of Tests and Criteria”, 5th Revised Edition, 2009, 2 test methods for determining the deflagration capability are described in section 23 (p. 237 ff).
In the test C.1 (“Time/Pressure Test”), 5 g of the substance to be tested are ignited in a pressure vessel having a capacity of about 17 ml. Criteria for the evaluation are attainment of a limit pressure of about 20.7 bar gauge and also the time after ignition in which the limit pressure is reached. (Bar gauge=bar gauge pressure)
The deflagration capability is assessed as follows in the test C.1:                yes, capable of fast deflagration, when the pressure within the pressure vessel increases from 6.90 bar gauge to 20.70 bar gauge in less than 30 seconds after ignition.        yes, capable of slow deflagration, when the pressure within the pressure vessel increases from 6.90 bar gauge to 20.70 bar gauge in 30 seconds or more after ignition,        not capable of deflagration when the limit pressure of 20.70 bar gauge is not reached.        
In the test C.2, a sample is introduced into a Dewar vessel having an internal diameter of about 48 mm and a height of 180-200 mm. The mixture is ignited by means of an open flame.
The deflagration capability is assessed as follows in the test C.2:                yes, capable of fast deflagration, when the deflagration velocity is greater than 5 mm/sec.        yes, capable of slow deflagration, when the deflagration velocity is in the range from 0.35 mm/sec to 5 mm/sec.        not capable of deflagration when the deflagration velocity is less than 0.35 mm/sec or the reaction stops before reaching the lower mark.        
Overall, a substance is classed as not capable of deflagration when the substance was not classified as “capable of fast deflagration” in the test C.1 and was classified as not capable of deflagration in the test C.2.
A further test for determining the deflagration capability is described in VDI2263-1 (1990, p. 13 ff.).
In the test in accordance with VDI2263-1, a substance is introduced into a glass tube which has a diameter of about 5 cm and is closed at the bottom and in which a plurality of thermocouples are installed radially offset at various heights. After local initiation by means of a glow coil, a glow plug, a microburner or an ignition mixture of lead (IV) oxide and silicon, the propagation of the decomposition is determined. Initiation is effected from above and from the bottom of the bed. If the decomposition spreads in at least one of the experiments (ignition from above and ignition from below), the material is classified as capable of deflagration.
As ignition sources, it is possible to use, as alternative a glow coil, a glow plug, a microburner or an ignition mixture (silicon/lead oxide in a ratio of 3:2). The time of application and the energy input of the ignition sources are not defined further.
In the standard procedure in accordance with VDI2263-1, the deflagration behaviour is measured at ambient temperature pressure. However, it can also be measured at elevated temperature and in a closed vessel.
It is known that many materials decompose without formation of a closed front and also incompletely in the test in accordance with VDI2263-1. Within the bed, there is frequently formation of channels in the interior of which the decomposition progresses while the surrounding material does not decompose. However, such behaviour represents a hazard potential for processing of a material. A person skilled in the art will select the parameters for the testing of the deflagration behaviour of a material or mixture in such a way that the situation during processing is most accurately reproduced. Thus, in the test in accordance with VDI2263-1, a substance will be brought to the temperature at which processing of the substance is also carried out. As regards the source of ignition, it can be assumed that the substance is not capable of deflagration when no propagation of the reaction is observed after 300 seconds of application of the source of ignition at a temperature of >600° C., for example by means of a glow coil or a glow plug, the latter at an energy input of 40 W. In the case of propagation of the reaction, any type of continuation of the decomposition which propagates through the bed should be evaluated as a sign of deflagration behaviour, even when channel formation is present and the bed does not react over its full width to form a decomposition front.
VDI report 975 (1992) page 99 ff, describes a classification of pulverulent materials which pose a deflagration hazard. The materials capable of deflagration are divided into 3 hazard classes. While materials of the hazard class 3 are in principle not allowed to be processed in apparatuses having mechanical internals, materials of the hazard classes 1 and 2 can be processed in apparatuses having mechanical internals subject to particular provisos.
Important criteria for classification into one of the 3 hazard classes is the plug action time, i.e. the time for which the ignition source in the test VDI2263-1 is switched on from when it is first switched on until the decomposition reaction becomes visible. The authors compare the plug action time with the minimum ignition energy in the case of dust explosions. The plug action time can, with a view to processing in a production apparatus, also be interpreted as the period of time for which an ignition source such as a hot starting place or a hot screw can act on the surrounding substance before a noncritical state is reached again by cooling of the starting place or screw or renewal of the environment around the hot place. Thus: “the shorter the plug action time, the easier it is to trigger deflagration”. The authors indicate a plug action time of ≦20 seconds as limit value for classification in hazard class 3 and a limit value of >60 seconds as limit value for classification in hazard class 1.
The production of solids capable of deflagration is carried out using the conventional process steps known from organic and inorganic chemistry. Starting materials are usually reacted with one another in liquid form or in the form of solutions, and the desired material usually precipitates as solid. This is then separated off from the remaining liquid components and is, after further possible purification steps, drying and temporary storage, available in the desired form for packaging and transport to the users. The desired material is optionally processed further and, for example, milled and/or mixed with other components.
The production of solids capable of deflagration is generally unproblematic on the laboratory scale. The amounts handles are small, the probability of initiation of deflagration is low, any deflagration occurring is quickly recognized and even in the case of nonrecognition and propagation of deflagration, the degree of damage is small.
However, the production of materials capable of deflagration is problematical in the case of relatively large amounts as are encountered in a pilot plant operation or production operation. Here, a series of apparatuses, which each have potential initiation sources and in the case of which deflagration can sometimes only be detected a relatively long time after initiation, are used.
Apparatuses in pilot plants and production operations are frequently equipped with mechanical devices which serve for transport, mixing, renewal of the surface or other purposes.
Thus, for example, mixers having moving mechanical elements, for example ploughshare mixers or screw mixers, are used for the homogenization of solids. It is known that the mechanical devices are one of the most frequent causes of initiation of deflagration. Thus, in the case of a malfunction, a moving mixing element can come into direct contact with the wall of the apparatus and local heating occurs at the point of friction and this heating can induce the surrounding material to decompose and thus initiate deflagration. Cases in which a foreign body, for example a screw, has got into an apparatus, got between the wall and stirring/mixing element there and triggered deflagration as a result of heating are likewise known. Even rubbing of hard crusts or friction in a blocked transport screw has resulted in triggering of deflagrations. It is also known that deflagrations can be transferred from one apparatus to the other. Thus, a screw which has been introduced into a mixer can be heated by friction in the manner described. The hot screw is then discharged, for example, into a silo without mechanical internals. The temperature of the screw can still be sufficiently high to induce the surrounding substance to decompose in the silo and thus trigger deflagration. In the same way, agglomerates in which deflagrative decomposition has been triggered can be discharged into an apparatus without mechanical internals and there initiate the deflagrative decomposition of the contents of the apparatus.
A series of measures which allow safe processing of materials capable of deflagration are known.
VDI report 975 (1992), page 99 ff, sets out a methodology for assessing and selecting measures in the processing of pulverulent materials which represent a deflagration hazard. The report describes classification of the materials capable of deflagration into 3 hazard classes, with materials in the hazard class 3 having the highest hazard potential and materials in the hazard class 1 having the lowest hazard potential. Various processing methods are indicated according to the hazard class. Although the criteria mentioned in the said publication do not have general validity, the methodology set out in this publication represents a good starting point for assessing and processing materials capable of deflagration. Examples of safe processing of materials capable of deflagration may also be found in the VDI report 1272 (1996), page 441 ff. In the case of materials having a high deflagration tendency, it is ensured that processing is carried out without mechanical action. This is achieved, for example, by drying being carried out on individual trays in a drying oven rather than in a dryer having mechanical internals, for example a paddle dryer. However, processing without mechanical devices is very complicated. The transport of material frequently has to be carried out manually, which can lead not only to high costs but also to hazards to the health of the operating personnel and to quality problems. Processing without mechanical devices will come into question only when safe processing using mechanical devices is not possible. For example, in the above-cited publication in the VDI report 975 (1992), page 99 ff, only processing methods without mechanical devices are allowed for the materials of hazard class 3.
In the case of materials for which the hazard potential posed by deflagration is less pronounced, mechanical devices can also be used for processing subject to particular conditions. In the cited publication in the VDI report 975 (1992), page 99 ff, this applies to materials in hazard classes 1 and 2.
One customary method of avoiding deflagration is the careful avoidance of introduction of foreign bodies. This can, for example, be effected by removal of metal before introduction in the apparatus so as to prevent the carrying-through of screws and other metallic foreign bodies into the processing step.
Even in the construction of the apparatuses, attention can be paid to avoidance of possible ignition sources, for example by selecting large spacings between a mechanical mixer and the wall.
The abovementioned methods of avoiding sources of ignition can significantly reduce the risk of deflagration, but deflagration can also be ruled out thereby. The methods mentioned are also complicated and in some cases associated with an impairment of the performance of the apparatuses.
A further possible way of avoiding deflagration is to mix the substance capable of deflagration with a further substance which is not capable of deflagration and does not have catalytic activity. A disadvantage of this measure is that the desired substance cannot be obtained with the desired composition. The reduction in the deflagration capability by addition of a further substance is described, for example, in U.S. Pat. No. 5,268,177.
A further method of safely processing substances capable of deflagration is to safely release the pressure arising in a deflagration or safely discharge the gases formed in the deflagration. This can, for example, be achieved by installation of appropriately dimensioned bursting discs and appropriate discharge devices. It has to be noted here that the deflagration velocity increases with increasing pressure, and actuation pressure and discharge line have to be designed accordingly. It also has been noted that entrained substances have to be hindered from propagating the deflagration. This can, for example, be achieved by introducing the discharge gases into a water bath.
A further known method of safely processing substances capable of deflagration is to recognize the commencement of deflagration in good time and suppress the incipient deflagration by removal of the energy. Recognition can be achieved via a series of indicators. For example, the monitoring of temperature and/or pressure is known. However, detection can also be effected via occurrence of particular decomposition gases such as carbon monoxide. When the trigger value has been reached, the energy is removed from the system. In general, this is effected by addition of a relatively large amount of water. The deflagrating substance is cooled to temperatures below the decomposition temperature by the heat capacity of the water. Additional removal of heat can be effected by the formation of water vapour. A detergent can be added to the water in order to ensure good wetting of the deflagrating substance.
A disadvantage of the abovementioned method is that they act only to limit damage and become effective only after triggering of the deflagration. These methods thus lead to loss of at least part of the substance, since the latter partly decomposes and the undecomposed proportions are generally made unusable by water and other reagents. The safe removal of water vapour formed is also problematical.
It can be stated that the methods described hitherto for processing substances capable of deflagration have disadvantages.
It was therefore an object of the present invention to provide better measures for processing and/or handling solids or solid mixtures capable of deflagration. In particular, these measures should reduce the probability of triggering of deflagration without altering the materials properties by addition of a further material.